Phosphonate compounds have long been known to provide a variety of therapeutic benefits. A particular class of therapeutically beneficial phosphonate compounds are the bisphosphonates, i.e., pyrophosphate analogs wherein the central oxygen atom of the pyrophosphate bond is replaced by carbon. Various substituent groups may be attached to this central carbon atom to produce derivative bisphosphonate compounds having various degrees of pharmacological potency. These derivatives have the general structure:
wherein Ra and Rb may independently be selected from hydroxyl, amino, sulfhydryl, halogen, or a variety of alkyl or aryl groups, or a combination of such groups, which may be further substituted. Examples include Etidronate, wherein Ra is CH3 and Rb is OH; Clodronate, dichloromethylene bisphosphonic acid (CI2MDP), wherein Ra and Rb are Cl, Pamidronate, 3-amino-1-hydroxypropylidene bisphosphonic acid, wherein Ra is ethylamino and Rb is hydroxyl; Alendronate, 4-amino-1-hydroxybutylidene bisphosphonic acid, wherein Ra is propylamino and Rb is hydroxyl; Olpadronate, 3-dimethylamino-1-hydroxypropylidene bisphosphonic acid, wherein Ra is dimethylaminoethyl and Rb is hydroxyl; and amino-olpadronate (IG-9402), 3-(N,N-dimethylamino)-1-aminopropylidene bisphosphonate, wherein Ra is N,N-dimethylaminoethyl and Rb is NH2.
Bisphosphonates and their substituted derivatives have the intrinsic property of inhibiting bone resorption in vivo. Bisphosphonates also act by inhibiting apoptosis (programmed cell death) in bone-forming cells. Indications for their use therefore include the treatment and prevention of osteoporosis, treatment of Paget's disease, metastatic bone cancers, hyperparathyroidism, rheumatoid arthritis, algodistrophy, stemo-costo-clavicular hyperostosis, Gaucher's disease, Engleman's disease, and certain non-skeletal disorders. (Papapoulos, S. E., in Osteoporosis, R. Marcus, D. Feldman and J. Kelsey, eds., Academic Press, San Diego, 1996. p. 1210, Table 1).
Although bisphosphonates have therapeutically beneficial properties, they suffer from pharmacological disadvantages as orally administered agents. One drawback is low oral availability: as little as 0.7% to 5% of an orally administered dose is absorbed from the gastrointestinal tract. Oral absorption is further reduced when taken with food. Further, it is known that some currently available bisphosphonates, e.g., FOSAMAX™ (Merck; alendronate sodium), SKELID™ (Sanofi, tiludronate) and ACTONE™ (Procter and Gamble, risedronate) have local toxicity, causing esophageal irritation and ulceration. Other bisphosphonates, like amino-olpadronate, lack anti-resorptive effects (Van Beek, E. et al., J. Bone Miner Res 11(10): 1492–1497 (1996) but inhibit osteocyte apoptosis and are able to stimulate net bone formation (Plotkin, L. et al., J Clin Invest 104(10): 1363–1374 (1999) and U.S. Pat. No. 5,885,973). It would therefore, be useful to develop chemically modified bisphosphonate derivatives that maintain or enhance the pharmacological activity of the parent compounds while eliminating or reducing their undesirable side effects.
In addition to bisphosphonates, monophosphonates are also known to provide therapeutic benefits. One class of therapeutically beneficial monophosphonates are the antiviral nucleotide phosphonates, such as, for example, cidofovir, cyclic cidofovir, adefovir, tenofovir, and the like, as well as the 5′-phosphonates and methylene phosphonates of azidothymidine, ganciclovir, acyclovir, and the like. In compounds of this type, the 5′-hydroxyl of the sugar moiety, or its equivalent in acyclic nucleosides (ganciclovir, penciclovir, acyclovir) which do not contain a complete sugar moiety, is replaced with a phosphorus-carbon bond. In the case of the methylene phosphonates, a methylene group replaces the 5′-hydroxyl or its equivalent, and its carbon atom is, in turn, covalently linked to the phosphonate. Various AZT structures are presented below, including compounds contemplated for use in the practice of the present invention. AZT itself is shown on the left. Compound A is AZT-monophosphate which has the usual phosphodiester link between the sugar and the phosphate. In contrast, in compounds B (AZT 5′-phosphonate) and C (AZT 5′-methylene phosphonate), the 5′-hydroxyl of 3′-azido, 2′,3′-dideoxyribose is absent and has been replaced by either a phosphorus-carbon bond (AZT phosphonate) or by a methylene linked by a phosphorus-carbon bond (AZT methylene phosphonate). Compounds B and C are examples of compounds useful in the practice of the present invention.

Compounds of this type may be active as antiproliferative or antiviral nucleotides. Upon cellular metabolism, two additional phosphorylations occur to form the nucleotide phosphonate diphosphate which represents the equivalent of nucleoside triphosphates. Antiviral nucleotide phosphonate diphosphates are selective inhibitors of viral RNA or DNA polymerases or reverse transcriptases. That is to say, their inhibitory action on viral polymerases is much greater than their degree of inhibition of mammalian cell DNA polymerases α, β, and γ or mammalian RNA polymerases. Conversely, the antiproliferative nucleotide phosphonate diphosphates inhibit cancer cell DNA and RNA polymerases and may show much lower selectivity versus normal cellular DNA and RNA polymerases. Since nucleotide phosphonates are poorly absorbed from the GI tract they frequently require parenteral administration (e.g. cidofovir). Furthermore, the negatively charged phosphonate moiety may interfere with cellular penetration, resulting in reduced activity as antivirals or antiproliferatives. Invention compounds may surprisingly overcome the disadvantages of this class of agents.
Pharmacologically active agents of antiviral phosphonates are known; the following U.S. Patents describe other approaches for nucleotide phosphonate analogs: U.S. Pat. No. 5,672,697 (Nuleoside-5′-methylene phosphonates), U.S. Pat. No. 5,922,695 (Antiviral phosphonomethoxy nucleotide analogs), U.S. Pat. No. 5,977,089 (Antiviral phosphonomethoxy nucleotide analogs), U.S. Pat. No. 6,043,230 (Antiviral phosphonomethoxy nucleotide analogs), U.S. Pat. No. 6,069,249. The preparation and use of alkylglycerol phosphates covalently linked to non-phosphonate containing drugs having amino, carboxyl, hydroxyl or sulfhydryl functional groups have previously been disclosed. These prodrugs optionally comprise a linker group or one or two additional phosphates esters between the drug and the alkyl glycerol phosphate (U.S. Pat. No. 5,411,947 and U.S. patent application Ser. No. 08/487,081). Partial esters of chloromethanediphosphonic acid are known (U.S. Pat. No. 5,376,649) and dianhydrides of clodronate have been reported (Ahlmark, et al., J Med Chem 42: 1473–1476 (1999)). However, the partial esters were found to not release the active bisphosphonate by chemical or biochemical conversion (Niemi, R. et al, J Chrom B 701:97–102 (1997)). Prodrugs comprising alkylglycerol phosphate residues attached to antiviral nucleosides (U.S. Pat. No. 5,223,263) or phosphono-carboxylates (U.S. Pat. No. 5,463,092) have also been described.
There is, therefore, a continuing need for less toxic, more effective pharmaceutical agents to treat a variety of disorders, such as those caused by viral infection and inappropriate cell proliferation, e.g., cancer. Thus, it is an object of the present invention to develop chemically modified phosphonate derivatives of pharmacologically active agents, e.g., antiviral and anti-neoplastic pharmaceutical agents. These modified derivatives increase the potency of the parent compound while minimizing deleterious side effects when administered to a subject in need thereof.